FWIW, if we do go this route, it’d be friendlier IMO to use some sort of legible names for the features instead (eg: upload-times, versions) rather than using numbers instead. Besides, 691 is implied. 
See also Consider renaming this project to not include the PEP number · Issue #10 · FFY00/python-pyproject-metadata · GitHub, Rename `pep517` to `pyproject-hooks`, and trim functionality · Issue #136 · pypa/pyproject-hooks · GitHub and Name for pyproject.toml builds and PEPs about not using PEP numbers for user-facing functional pieces (yes, this is a format detail but still – similar argues apply since the consumers are written by people 
).